The present invention is generally directed to a mechanical system having a rotary drive speed-torque conversion device. More particularly, the present invention is directed to a robotic harmonic flex-drive.
Conventional modular robotic systems enable single degree-of-freedom rotary joint modules in arrangements suited for specific automation tasks. Each joint module contains a control processor, a drive motor and a speed reduction gear mechanism. These devices all work together in coordination with other joints to move a payload along a specified trajectory with precisely controlled position, velocity and torque. The reduction gear mechanism has previously been provided by commercial harmonic drive assemblies and is illustrated in FIGS. 1, 2 and 3A-3C. Several important properties of such a harmonic drive assembly includes small size, moderate weight, high efficiency, zero backlash, and high torque.
FIGS. 1, 2 and 3A-3C illustrate a conventional harmonic drive assembly 10 which includes three major components: a circular spline 12; a flex spline 14; and a wave generator 16. The circular spline 12 is a rigid circular structure with a set of gear teeth 18 disposed along the inside diameter of the circular spline 12. The flex spline 14 is a cup-shaped structure with a set of gear teeth 20 around an upper outside surface 22 of the flex spline 14. As shown in FIG. 2, the flex spline 14 further includes a tube 24, a diaphragm 26 and a flange 28. The wave generator 16 is an elliptical ball bearing assembly having a plurality of ball bearings 30, a hub 32, a plug 34 and an Oldham coupling 36. The flex spline 14 is positioned coaxially inside of the circular spline 12, which has a larger inside diameter than the outside diameter of the flex spline 14. The circular spline 12 has two more gear teeth 18 relative to the quantity of gear teeth 20 of the flex spline 14.
The elliptical wave generator 16 is also positioned on a common axis 38 with the circular spline 12 and the flex spline 14. The wave generator 16 fits inside the flex spline 14 and has a major axis width 40 (FIGS. 3A-3C) sufficient to deflect an upper edge 42 and a lower edge 44 of the flex spline 14 outward, causing the outside gear teeth 20 of the flex spline 14 to engage the inside gear teeth 18 of the circular spline 12 at two diametrically opposite contact points 46, 48. The wave generator 16 is typically driven by an electric motor, not shown. Rotation of the wave generator 16 causes simultaneous rotation of the flex spline 14 and the engaged circular spline 12 via the engaged gear teeth 18, 20. Although, the flex spline 14 and the wave generator 16 rotate at different speeds relative to one another. The difference between the quantity of the inside gear teeth 18 of the circular spline 12 relative to the outside gear teeth 20 of the flex spline 14 causes a rotational displacement between the circular spline 12 and the flex spline 14. In general, for a circular spline 12 that has N inner gear teeth 18, the wave generator 16 needs to rotate N/2 times to cause one complete rotation of the flex spline 14. Thus, the conventional harmonic drive assembly 10 provides a speed reduction of N/2 from the wave generator 16 to the flex spline 14. Such a reduction in speed induces a corresponding increase in torque within the harmonic drive assembly 10.
The core principle of operation of the conventional harmonic drive assembly 10 derives from the deformed cylindrical geometry of the flex spline 14, not from the engagement of the inside gear teeth 18 of the circular spline 12 with the outside gear teeth 20 of the flex spline 14. The engagement of the gear teeth 18, 20 only serves to prevent slippage to prolong the life span of the harmonic drive assembly 10 and to provide more efficient operation. The degree of deflection in the flex spine 14 as illustrated in FIGS. 3A-3C is exaggerated to demonstrate the operation of the harmonic drive assembly 10. The actual deflection is much smaller than shown and is well within the material fatigue limits for infinite service life. As shown in FIG. 3A, the elliptical wave generator 16 deflects the upper edge 42 of the flex spline 14 such that the outside gear teeth 20 of the flex spline 14 engage the inside gear teeth 18 of the circular spline 12 along the major axis 40 at the diametrically opposed contact points 46, 48. The inside gear teeth 18 of the circular spline 12 are fully disengaged from the outside gear teeth 20 of the flex spline 14 along a minor axis 50. Most of the relative motion between the inside gear teeth 18 and the outside gear teeth 20 occurs along the minor axis 50. The rigid circular spline 12 is commonly rotationally fixed. FIGS. 3B and 3C illustrate rotation of the wave generator 16 by ninety and one hundred eighty degrees, respectively. An example of a conventional harmonic drive assembly is illustrated in U.S. Pat. No. 2,906,143 to Musser.
The primary failure mechanism of the conventional harmonic drive assembly 10 derives from the rigidity of the circular spline 12, which, under heavy load, deforms enough to allow gear tooth slippage. Slippage also increases the risk of backlash. Gear slippage is therefore often destructive to the gear teeth 18, 20. This problem has been addressed by increasing the thickness of the circular spline 12. Consequently, and undesirably, increased thickness significantly increases the weight and size of the entire harmonic drive assembly 10. Additionally, increased thickness increases the cost of the components, which are already expensive to fabricate. These additional costs significantly increase the manufacturing cost of the entire robotic arm assembly.
Accordingly, there is a need for a harmonic drive assembly that significantly improves on the prior art described above. Such a harmonic drive assembly should improve on properties that include, without limitation, small size, moderate weight, high efficiency, zero backlash, high torque and the like. Such a harmonic drive assembly should include an inner spline with outside gear teeth and an outer spline with inside gear teeth that are engaged by a rotational wave generator having multiple roller bearings for deflecting the splines together. Accordingly, such a harmonic drive assembly can be smaller in size, have decreased weight, have an increased efficiency, include zero backlash, and be capable of receiving higher torques without risk of failure. The present invention fulfills these needs and provides other related advantages.